The present invention relates to a cathode ray tube (CRT), and particularly to a method of correcting deflection defocusing in a cathode ray tube which is capable of improving focus characteristics and thereby obtaining a sufficient resolution over the entire phosphor screen and over the entire electron beam current region; a cathode ray tube employing the method; and an image display system including the cathode ray tube.
A cathode ray tube such as a picture tube or a display tube includes at least an electron gun having a plurality of electrodes and a phosphor screen (screen having a phosphor film, which is also referred to as "phosphor film" or simply as "screen" hereinafter), and it also includes a deflection device for allowing an electron beam emitted from the electron gun to scan on the phosphor screen.
As for such a cathode ray tube, there have been known the following techniques for obtaining a desirable reproduced image on the entire phosphor screen from the center to the peripheral portion.
Japanese Patent Publication No. Hei 4-52586 discloses an electron gun emitting three in-line electron beams in which a pair of parallel flat electrodes are disposed on the bottom face of a shield cup in such a manner as to be positioned above and below paths of the three electron beams in parallel to the in-line direction and to extend toward a main lens.
U.S. Pat. No. 4,086,513 and its corresponding Japanese Patent Publication No. Sho 60-7345 disclose an electron gun emitting three in-line electron beams in which a pair of parallel flat electrodes are disposed above and below paths of the three electron beams in parallel to the in-line direction in such a manner as to extend from a facing end of one of a pair of main-lens-forming electrodes toward a phosphor screen, thereby shaping the electron beams before the electron beams enter a deflection magnetic field.
Japanese Patent Laid-open No. Sho 51-61766 discloses an electron gun in which an electrostatic quardrupole lens is formed between two electrodes and the strength of the electrostatic quardrupole lens is made to vary dynamically in synchronization with the deflection of an electron beam, thereby achieving uniformity of an image over the entire screen.
Japanese Patent Publication No. Sho 53-18866 discloses an electron gun in which an astigmatic lens is provided in a region between a second grid electrode and a third grid electrode forming a prefocus lens.
U.S. Pat. No. 3,952,224 and its corresponding Japanese Patent Laid-open No. Sho 51-64368 discloses an electron gun emitting three in-line electron beams in which an electron beam aperture of each of first and second grid electrodes is formed in an elliptic shape, and the degree of ellipticity of the aperture is made to differ for each beam path or the degree of ellipticity of the electron beam aperture of the center electron gun is made smaller than that of the side electron gun.
Japanese Patent Laid-open No. Sho 60-81736 discloses an electron gun emitting three in-line electron beams in which a slit recess provided in a third grid electrode on the cathode side forms a non-axially-symmetrical lens, and an electron beam is made to impinge on the phosphor screen through at least one non-axially-symmetrical lens in which the axial depth of the slit recess is larger for the center beam than for the side beam.
Japanese Patent Laid-open No. Sho 54-139372 discloses a color cathode ray tube having an electron gun emitting three in-line electron beams in which a soft magnetic material is disposed in fringe portions of the deflection magnetic field to form a pincushion-shaped magnetic field for deflecting the electron beams in the direction perpendicular to the in-line direction of each electron beam, thereby suppressing a halo caused by the deflection magnetic field in the direction perpendicular to the in-line direction.
The desirable focus characteristics of a cathode ray tube include a desirable resolution over the entire screen and over the entire electron beam current region; a characteristic without generation of moire in a small-current region; and uniformity in resolution over the entire screen and over the entire electron beam current region. The design of an electron gun for simultaneously satisfying a plurality of these focus characteristics requires a high technique.
The studies by the present inventors showed that an electron gun having a combination of an astigmatic lens and a large-diameter main lens is essential to give the above focus characteristics to a cathode ray tube.
In the above-described related arts, however, a dynamic focus voltage has been required to be applied to a focus electrode of an electron gun for obtaining a desirable resolution over the entire screen using electrodes forming an astigmatic lens, that is, non-axially-symmetrical lens in the electron gun.
FIG. 80 is a side view of the entire configuration of one example of an electron gun used for a cathode ray tube; and FIG. 81 is a partial sectional view seen in the direction of an arrow of FIG. 80 showing an essential portion of the electron gun.
The electron gun of this type has a plurality of electrodes including a cathode K, a first grid electrode (G1) 1, a second grid electrode (G2) 2, a third grid electrode (G3) 3, a fourth grid electrode (G4) 4, a fifth grid electrode (G5) 5, a sixth grid electrode (G6) 6, and a shield cup 100 integrally attached to the sixth grid electrode (G6) 6. In addition, the fifth grid electrode (G5) 5 is composed of two electrodes 51, 52.
A focus voltage is applied between the third grid electrode 3 and the fifth electrode 5, and an anode voltage is applied only to the sixth electrode 6, so that an electron beam produced by a so-called triode portion composed of the cathode K, the first grid electrode 1 and the second grid electrode 2 is accelerated and focused by an electron lens formed by the third grid electrode 3 to the sixth grid electrode 6, to project toward a phosphor screen.
Effects on an electron beam of electric fields determined by lengths of the electrodes, and diameters of electron beam apertures in the electrodes of this electron gun differ from electrode to electrode. For example, the shape of the electron beam aperture of the first grid electrode near the cathode K exerts an effect on the spot shape of an electron beam in a small-current region; however, the shape of the electron beam aperture of the second grid electrode exerts an effect on the spot shape of an electron beam in a wide current region from the small-current region to the large-current region.
In the electron gun in which a main lens is formed between the fifth grid electrode 5 and the sixth grid electrode 6 by applying an anode voltage to the sixth grid electrode 6, the shape of the electron beam aperture of each of the fifth grid electrode 5 and the sixth grid electrode 6 forming the main lens exerts a large effect on the shape of the electron beam in a large-current region but exerts a smaller effect on the shape of the electron beam in a small-current region than in the large-current region.
The axial length of the fourth grid electrode 4 of the electron gun exerts an effect on the magnitude of the optimum focus voltage and also exerts a large effect on a difference in the optimum focus voltage between a small-current region and a large-current region. The effect of the axial length of the fifth grid electrode 5, however, is significantly smaller than that of the fourth grid electrode 4.
Accordingly, it is required for optimizing the characteristics of each electron beam to optimize the structure of each electrode to be most effective to each characteristic of the electron beam.
In the case where a shadow mask pitch in the direction perpendicular to the electron beam scanning direction is made smaller or the density of electron beam scanning lines is increased for enhancing resolution in the direction perpendicular to the electron beam scanning direction of a cathode ray tube, an interference is generated between the electron beam scanning line and the shadow mask particularly in the electron beam small-current region, and accordingly moire contrast must be suppressed. The technical developments in this art area, however, have yet to solve the above-described problems.
For example, FIG. 82A and 82B are schematic views, each showing an essential portion of an electron gun, for comparing the two structures of the electron guns depending on the manner of supplying the focus voltage with each other; wherein FIG. 82A shows a fixed-focus-voltage type electron gun; and FIG. 82B shows a dynamic-focus-voltage type electron gun.
The configuration of the electron gun of the fixed-focus-voltage type shown in FIG. 82A is the same as that shown in FIGS. 80 and 81, and therefore, parts corresponding to those in FIGS. 80 and 81 are indicated by the same characters.
In the electron gun of the fixed-focus-voltage type shown in FIG. 82A, a focus voltage Vf1 having the same potential is applied to the electrodes 51 and 52 forming the fifth grid electrode 5. In this figure, an equation of the opening radius R.sub.5 &gt;0.1.times.opening radius Rs is satisfied.
On the other hand, in the electron gun of the dynamic-focus-voltage type shown in FIG. 82B, different focus voltages are respectively supplied to the electrodes 51 and 52 forming the fifth grid electrode 5. In particular, a dynamic focus voltage dVf is supplied to the electrode 52.
In the electron gun of the dynamic-focus-voltage type shown in FIG. 82B, moreover, the electrode 52 has a portion extending in the electrode 51. This complicates the structure as compared with the electron gun shown in FIG. 82A, to increase the cost of parts and make poor the efficiency in the assembling process.
FIGS. 83A and 83B are graphs showing focus voltages respectively supplied to the electron guns shown in FIGS. 82A and 82B, wherein FIG. 83A shows a focus voltage supplied to the electron gun of the fixed-focus-voltage type; and FIG. 83B shows the focus voltage supplied to the electron gun of the dynamic-focus-voltage type.
Specifically, FIG. 83A shows the state that the fixed focus voltage Vf.sub.1 is applied to the third grid electrode 3 and the fifth grid electrode 5 (51, 52). On the other hand, FIG. 83B shows the state that the fixed focus voltage Vf.sub.1 is applied to the third electrode 3 and the electrode 51 of the fifth grid electrode 5 and a voltage having a waveform in which another fixed focus voltage Vf.sub.2, superposed with the dynamic focus voltage dVf, is applied to the electrode 52 of the fifth grid electrode 5.
As a result, the electron gun of the dynamic-focus-voltage type shown in FIG. 83B requires two stem pins for supplying focus voltages, and thereby it requires high-voltage insulation from the other stem pin as compared with the electron gun of the fixed-focus-voltage type shown in FIG. 83A.
Accordingly, the dynamic-focus-voltage type electron gun requires a specified structure in a current supply socket to a cathode ray tube in a TV receiver set and a terminal display system, and further it requires a dynamic-focus-voltage generating circuit in addition to the two fixed-focus-voltage power supplies. This causes a disadvantage in that it takes a lot of time for adjusting two focus voltages the lens actions of which interact with each other and phasing a dynamic focus voltage to electron beam deflection.
Especially, for use in multimedia expected to be widely spread soon, a display system needs to be capable of being driven at a plurality of deflection frequencies. This requires dynamic focus voltage generators for respective deflection frequencies and phasing a dynamic focus voltage to electron beam deflection at respective frequencies, increasing the cost of electrical circuits and set-up procedures, which which cost increases with the screen size and maximum deflection angle of a cathode ray tube exponentially